1. Field of the Invention
The present invention relates generally to the field of spectroscopy and particularly to Fourier transform Raman spectrometry.
2. Description of the Related Art
In Raman spectroscopy, a laser beam is directed onto a sample and the radiation reflected from the sample is collected, passed through an interferometer, and then detected. The reflected radiation is characterized by a strong component, known as the Rayleigh line, at the wavelength of the laser, with weaker secondary components, known as the "Stokes" and "anti-Stokes" lines, at wavelengths slightly above and below the laser wavelength. The Stokes and anti-Stokes lines are referred to collectively as the "Raman" lines. The objective in Raman spectroscopy is to analyze the Stokes and/or anti-Stokes lines for spectral information about the sample.
The Raman scattering effect is illustrated in FIG. 1. For example, using an Nd:YAG laser as the excitation source, the sample is raised to an excitation level equal to that of the source and it then decays in one of three possible ways. It can fall back to its original ground state, to the next higher energy state, or, if the original state was the first electronic state as expected by Boltzman's distribution, from the first electronic state to the ground state. These three possibilities result in the Rayleigh scattering, Stokes scattering, and anti-Stokes scattering, respectively.
A typical Raman spectrometer arrangement is shown in FIG. 2. The light beam 20 from a laser 21 is reflected by mirrors 22 and 23 to sample collection optics 25 which allows the beam 20 to strike the sample 27 and then collects the reflected radiation and collimates it into a beam 29. The beam of reflected radiation 29 is passed through a stack of filters 30 before the beam proceeds into an interferometer 32 (e.g., a Michelson interferometer shown schematically as having a beam splitter 34, a moving mirror 35 and a stationary mirror 36). After passing through the interferometer, the beam 37 is collected by optics shown schematically at 39 and focused onto a detector 40. To achieve sufficient attenuation of the Rayleigh line, the stack of filters 30, typically dielectric filters, produce attenuation of the Rayleigh line on the order of 10.sup.-6. Dielectric filters are conventionally edge filters, which attenuate either the Stokes or anti-Stokes line as well as the Rayleigh line. The throughput of the passband of the filter is reduced by each additional filter. A throughput of only 30% of the desired Raman wavelengths would be typical for a five filter stack typically required to achieve 10.sup.-6 attenuation of the Rayleigh line.
Recent developments in the aerospace industry have substantially increased the use of fiber-optic Raman spectroscopy to monitor the high temperature curing of high performance polymers. One approach which has been successful is the use of a dispersive Raman instrument using short-wavelength near-Infrared (IR) radiation (.about.800 nanometers). Although this approach is feasible for many polymer cures, polymers which are highly fluorescent still present an obstacle. In particular, modified polyimides represent the largest and most promising class of high-temperature polymers and yet cannot be analyzed with dispersive instruments due to their intense fluorescence. Because Fourier Transform (FT) Raman instruments have a longer wavelength excitation, they allow the acquisition of polyimide Raman spectra at room temperature, but these spectra cannot be acquired within an autoclave or high-temperature oven in real time due to the thermal background which occurs at the high temperatures required for processing the polyimide. This ability for real-time in-situ monitoring is crucial if intelligent feedback systems based on direct chemical information are to be designed for processing autoclaves.
Several researchers have addressed the problem of thermal backgrounds in FT-Raman spectra. Cutler et al have successfully removed thermal backgrounds by synchronizing a Q-switched pulsed laser to the interferometer reference laser fringe crossings and adjusting the A/D sampling of the spectrometer so that it coincides with the peak of the detector pulses. This approach has also been used in combination with a fast analog filter to improve signal-to-noise ratio and to discriminate against long lived backgrounds. A second improvement is the use of a ratioing circuit to minimize pulse-to-pulse fluctuations of the Q-switched laser and thus further improve the signal-to-noise ratio. Sakamoto and coworkers have developed an asynchronous method to eliminate thermal backgrounds using a Q-switched pulsed laser in combination with a gate circuit and a low pass filter which avoids the necessity to trigger the laser using the interferometer reference laser fringe crossings. One drawback of all of these approaches is that the fast modulation frequencies of the Q-switched laser require that slow mirror velocities be used, ultimately resulting in a longer acquisition time relative to a conventional FT-Raman spectrometry experiment. In addition, the short laser pulse widths (ranging from 7-100 ns) and thus high laser peak powers result in a greater probability of laser-induced sample heating.
Bennett has also described an approach using a diode-pumped pulsed laser in combination with a sample and hold circuit which is triggered off of the A/D converter. This approach can be performed either synchronously or asynchronously (with respect to the A/D sampling), however, either method requires specialty software to reconstruct the Raman spectrum. In addition, Bennett states that optimum performance is achieved at slow interferometer mirror velocities (0.1 cm/s) which results in longer acquisition times. Recently, Petty has described a method to eliminate thermal backgrounds using a modulated laser and a step-scan FT-Raman spectrometer. Although successful, the step-scan experiment requires much longer acquisition times when compared to a conventional FT-Raman spectrometer.
Although all of these approaches describe an increase in signal-to-noise relative to a conventional FT-Raman spectrometer, the comparison is not strictly valid because a conventional spectrometer could utilize much faster mirror velocities and acquire more spectra in an equivalent time period, which would result in a signal averaging improvement in the signal-to-noise ratio for the conventional spectrometer. In addition, all of the described methods require rather involved and costly modifications to a commercial continuous wave (CW) laser FT-Raman spectrometer. Moreover, none of these existing methods have been demonstrated to remove thermal backgrounds at temperatures of 300.degree. C. or greater.